CN105228741B

CN105228741B - Catalyst, and electrode catalyst layer, membrane electrode assembly, and fuel cell using same

Info

Publication number: CN105228741B
Application number: CN201480023165.9A
Authority: CN
Inventors: 秋月健; 大间敦史; 真塩彻也; 小野义隆; 高桥真一
Original assignee: Nippon Steel and Sumikin Chemical Co Ltd; Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd; Nippon Steel Chemical and Materials Co Ltd
Priority date: 2013-04-25
Filing date: 2014-04-14
Publication date: 2019-12-06
Anticipated expiration: 2034-04-14
Also published as: WO2014175105A1; EP2990105A1; JPWO2014175105A1; US20160072133A1; EP2990105B1; CA2910374A1; EP2990105A4; US9947934B2; CA2910374C; CN105228741A; JP6113837B2

Abstract

The purpose of the present invention is to provide a catalyst having excellent oxygen reduction reaction activity. The catalyst comprises a catalyst carrier and a metal catalyst supported on the catalyst carrier, wherein the specific surface area of the catalyst per unit weight of the carrier is 715m2/g or more, or the coverage of the metal catalyst with an electrolyte is less than 0.5, and the amount of acid groups per unit weight of the carrier in the catalyst is 0.75mmol/g or less.

Description

Catalyst, and electrode catalyst layer, membrane electrode assembly, and fuel cell using same

Technical Field

The present invention relates to a catalyst, particularly an electrode catalyst used for a fuel cell (PEFC), and an electrode catalyst layer, a membrane electrode assembly, and a fuel cell using the catalyst.

Background

A polymer electrolyte fuel cell using a proton conductive polymer electrolyte membrane operates at a lower temperature than other types of fuel cells such as a solid oxide fuel cell and a molten carbonate fuel cell. Therefore, the polymer electrolyte fuel cell is expected as a stationary power source or a motive power source for a mobile body such as an automobile, and practical application thereof is also being started.

As such a polymer electrolyte fuel cell, an expensive metal catalyst represented by Pt (platinum) or a Pt alloy is generally used, and this is a factor of increasing the price of such a fuel cell. Therefore, there is a demand for the development of a technique for reducing the amount of noble metal catalyst used and achieving cost reduction of the fuel cell.

For example, japanese patent application laid-open No. 2012-124001 (specification of U.S. patent application publication No. 2013/244137) discloses a catalyst for a polymer electrolyte fuel cell, in which catalyst particles made of platinum are supported on a carbon powder carrier. The carbon powder carrier is combined with 0.7-3.0 mmol/g (based on the weight of the carrier) of hydrophilic groups, the average particle diameter of the platinum particles is 3.5-8.0 nm, and the specific surface area (COMSA) of the CO adsorbed platinum is 40-100 m 2/g. Japanese unexamined patent application publication No. 2012-124001 (U.S. patent application publication No. 2013/244137) describes: the platinum catalyst having a deteriorated wettability by eliminating the functional group on the surface of the annealed carrier can ensure an initial activity (initial power generation characteristic) by introducing a hydrophilic group.

However, the catalyst containing disclosed in Japanese patent application laid-open No. 2012-124001 (U.S. patent application publication No. 2013/244137) has a problem that the activity of the oxygen reduction reaction is insufficient and the catalytic activity is decreased.

Disclosure of Invention

The present invention has been made in view of the above circumstances, and an object thereof is to provide a catalyst having excellent oxygen reduction reaction activity.

Another object of the present invention is to provide an electrode catalyst layer, a membrane electrode assembly, and a fuel cell having excellent power generation performance.

The present inventors have conducted extensive studies to solve the above problems, and as a result, have found that a catalyst having an acidic group in a predetermined amount or less can solve the above problems, and have completed the present invention.

Drawings

Fig. 1 is a schematic sectional view showing the basic structure of a polymer electrolyte fuel cell according to an embodiment of the present invention, in fig. 1, 1 is a Polymer Electrolyte Fuel Cell (PEFC), 2 is a polymer electrolyte membrane, 3a is an anode catalyst layer, 3b is a cathode catalyst layer, 4a is an anode gas diffusion layer, 4b is a cathode gas diffusion layer 4c, 5a is an anode separator, 5c is a cathode separator, 6a is an anode gas flow path, 6c is a cathode gas flow path, 7 is a refrigerant flow path, and 10 is a Membrane Electrode Assembly (MEA);

Fig. 2 is a schematic sectional explanatory view showing the shape and structure of the catalyst of the present invention, in fig. 2, 20 is the catalyst, 22 is the metal catalyst, 23 is the carrier, 24 is the hole (fine pore), 25 is the acid group, and 26 is the electrolyte.

Detailed Description

The catalyst of the present invention (also referred to as "electrode catalyst" in the present specification) is composed of a catalyst carrier and a metal catalyst supported on the catalyst carrier. Here, the catalyst satisfies the following structures (a) to (b):

(a) the specific surface area per unit weight of the carrier of the catalyst is 715m2/g or more;

(b) The amount of the acidic group per unit weight of the carrier in the above catalyst is 0.75mmol/g or less per the carrier.

The catalyst of the present invention comprises a catalyst carrier and a metal catalyst supported on the catalyst carrier. Here, the catalyst satisfies the following structures (c) and (b):

(c) The coverage rate of the electrolyte to the metal catalyst is less than 0.5;

(d) The amount of the acidic group per unit weight of the carrier in the above catalyst is 0.75mmol/g or less per the carrier.

In the present specification, holes having a radius of less than 1nm are also referred to as "micro (micro) holes". In the present specification, a hole having a radius of 1nm or more is also referred to as a "fine (meso) pore".

The present inventors have found that when the specific surface area of the carrier of the catalyst described in patent document 1 is increased, the metal catalyst is easily surrounded by an electrolyte (electrolyte polymer) or water, and the oxygen reduction reaction activity is lowered. In contrast, the present inventors have found that the amount of the electrolyte or water present on the surface of the metal catalyst can be reduced by reducing the amount of the acidic groups present in the catalyst and suppressing the coverage of the metal catalyst with the electrolyte, thereby improving the oxygen reduction reaction activity and the catalytic activity.

According to the present invention, by increasing the specific surface area of the carrier, the metal catalyst can be easily supported in the cavities of the carrier, and the coverage of the electrolyte on the surface of the metal catalyst can be suppressed. In addition, even if the coverage of the electrolyte is reduced, the coverage of the electrolyte on the surface of the metal catalyst can be suppressed. Further, by reducing the amount of acidic groups in the catalyst, the amount of water absorbed inside the pores of the catalyst can be suppressed, and the amount of water present in the vicinity of the metal catalyst can be reduced. Therefore, the catalyst of the present invention can exert high catalytic activity having high oxygen reduction reaction activity, that is, can promote a catalytic reaction. Therefore, a membrane electrode assembly and a fuel cell having a catalyst layer using the catalyst of the present invention have excellent power generation performance.

An embodiment of the catalyst of the present invention, and an embodiment of a catalyst layer, a Membrane Electrode Assembly (MEA), and a fuel cell using the catalyst will be described in detail below with reference to the drawings as appropriate. However, the present invention is not limited to the following embodiments. In addition, the drawings are exaggerated for convenience of explanation, and the dimensional ratios of the constituent elements in the drawings may be different from actual ones. In addition, when the embodiments of the present invention are described with reference to the drawings, the same reference numerals are attached to the same elements in the description of the drawings, and redundant description is omitted.

In the present specification, "X to Y" indicating a range means "X to Y" and "weight" and "mass", "weight%" and "mass%" and "part by weight" and "part by mass" are treated as synonyms. Unless otherwise specified, the operation and the measurement of physical properties are carried out under the conditions of room temperature (20 to 25 ℃) and relative humidity of 40 to 50%.

[ Fuel cell ]

The fuel cell includes a Membrane Electrode Assembly (MEA) and a pair of separators including an anode-side separator having a fuel gas flow field through which a fuel gas flows and a cathode-side separator having an oxidant gas flow field through which an oxidant gas flows. The fuel cell of the present embodiment has excellent durability and can exhibit high power generation performance.

Fig. 1 is a schematic diagram showing a basic structure of a Polymer Electrolyte Fuel Cell (PEFC)1 according to an embodiment of the present invention. First, the PEFC1 includes the solid polymer electrolyte membrane 2 and a pair of catalyst layers (an anode catalyst layer 3a and a cathode catalyst layer 3c) sandwiching the solid polymer electrolyte membrane 2. The laminate of the solid polymer electrolyte membrane 2 and the catalyst layers (3a, 3c) is further sandwiched between a pair of Gas Diffusion Layers (GDLs) (an anode gas diffusion layer 4a and a cathode gas diffusion layer 4 c). Thus, the solid polymer electrolyte membrane 2, the pair of catalyst layers (3a, 3c), and the pair of gas diffusion layers (4a, 4c) are stacked together to form a Membrane Electrode Assembly (MEA) 10.

in the PEFC1, the MEA10 is further sandwiched between a pair of separators (an anode separator 5a and a cathode separator 5 c). In fig. 1, separators (5a, 5c) are illustrated as being located at both ends of the illustrated MEA 10. In a fuel cell stack in which a plurality of MEAs are stacked, the separator is generally used as a separator for an adjacent PEFC (not shown). In other words, in the fuel cell stack, the MEAs are stacked in order via the separators to constitute the stack. In the actual fuel cell stack, gas seal portions are disposed between the separators (5a, 5c) and the solid polymer electrolyte membrane 2, or between the PEFC1 and another PEFC adjacent thereto, but these are not shown in fig. 1.

The separators (5a, 5c) are obtained by, for example, subjecting a thin plate having a thickness of 0.5mm or less to a press treatment to form the separator into an uneven shape as shown in fig. 1. The convex portions of the separators (5a, 5c) as viewed from the MEA side contact the MEA 10. Thereby, electrical connection to the MEA10 is ensured. In addition, the concave portions of the separators (5a, 5c) as viewed from the MEA side (the space between the separator and the MEA due to the uneven shape of the separator) function as gas flow paths for flowing gas during operation of the PEFC 1. Specifically, a fuel gas (e.g., hydrogen) is flowed through the gas channels 6a of the anode separator 5a, and an oxidizing gas (e.g., air) is flowed through the gas channels 6c of the cathode separator 5 c.

On the other hand, the recessed portions of the separators (5a, 5c) viewed from the side opposite to the MEA side are provided as the refrigerant flow paths 7 for allowing a refrigerant (e.g., water) for cooling the PEFC to flow therethrough during operation of the PEFC 1. Further, a manifold (not shown) is usually provided in the separator. The manifold functions as a coupling device for coupling the cells when the battery pack is configured. By adopting such a structure, the mechanical strength of the fuel cell stack can be ensured.

In the embodiment shown in fig. 1, the separators (5a, 5c) are formed in a concave-convex shape. The separator is not limited to such a shape having a concave-convex shape, and may be in any shape such as a flat plate shape or a shape having a partial concave-convex shape as long as the separator can function as a gas flow path and a refrigerant flow path.

the fuel cell having the MEA of the present invention as described above exerts excellent power generation performance. Here, the type of the fuel cell is not particularly limited, and the polymer electrolyte fuel cell is exemplified in the above description, but in addition to these, an alkaline fuel cell, a direct methanol fuel cell, a micro fuel cell, and the like can be cited. Among them, Polymer Electrolyte Fuel Cells (PEFC) are preferred from the viewpoint of their compact size, high density and high output. The fuel cell is used as a power source for a mobile body such as a vehicle defining a mounting space, and is also used as a power source for a stationary body. Among them, it is particularly preferable to be used as a power source for mobile bodies such as automobiles which request a high output voltage after a relatively long-time operation stop.

the fuel used for operating the fuel cell is not particularly limited. For example, the following may be used: hydrogen, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, sec-butanol, tert-butanol, dimethyl ether, diethyl ether, ethylene glycol, diethylene glycol, and the like. Among them, hydrogen or methanol is preferably used in order to achieve high output.

The application of the fuel cell is not particularly limited, but the fuel cell is preferably applied to a vehicle. The electrolyte membrane-electrode assembly of the present invention is excellent in power generation performance and durability, and can be miniaturized. Therefore, the fuel cell of the present invention is particularly advantageous in the case where the fuel cell is applied to a vehicle in terms of vehicle-mounted performance.

hereinafter, the components constituting the fuel cell of the present embodiment will be briefly described, but the technical scope of the present invention is not limited to the following embodiments.

[ catalyst (electrode catalyst) ]

Fig. 2 is a schematic cross-sectional explanatory view showing the shape and structure of a catalyst according to an embodiment of the present invention. As shown in fig. 2, the catalyst 20 of the present invention is composed of a metal catalyst 22 and a catalyst support 23. In addition, the catalyst 20 has a cavity (fine pore) 24. Further, the catalyst 20 has an acid group 25. Here, the metal catalyst 22 is supported inside the cavity (fine pore) 24. The metal catalyst 22 may be supported at least partially on the inside of the pores 24, or may be supported partially on the surface of the catalyst carrier 23. However, from the viewpoint of preventing contact between the electrolyte of the catalyst layer and the metal catalyst, it is preferable that substantially all of the metal catalyst 22 be supported inside the fine pores 24. Here, "substantially all of the metal catalyst" is not particularly limited as long as it is an amount capable of improving a sufficient catalytic activity. The "substantially all of the metal catalyst" is preferably present in an amount of 50% by weight or more (upper limit: 100% by weight), more preferably 80% by weight or more (upper limit: 100% by weight) of the total metal catalyst.

The BET specific surface area (of the catalyst after the metal catalyst is supported) [ BET specific surface area of the catalyst per 1g of the carrier (m2/g carrier) ] is not particularly limited, but is preferably at least 715m2/g of the carrier, more preferably at least 1200m2/g of the carrier, and particularly preferably at least 1700m2/g of the carrier. If the specific surface area is as described above, a sufficient number of pores (fine pores) can be secured, and a larger amount of the metal catalyst can be stored (supported) in the pores (fine pores). The covering of the metal catalyst with the electrolyte in the catalyst layer can be suppressed (the contact between the metal catalyst and the electrolyte can be suppressed and prevented more effectively). Therefore, the activity of the metal catalyst can be more effectively utilized to further effectively promote the catalytic reaction. The upper limit of the specific surface area is not particularly limited, but is preferably 3000m2/g or less.

Further, in the present specification, the "BET specific surface area (m2/g support)" of the catalyst is measured by a nitrogen adsorption method. Specifically, about 0.04 to 0.07g of the catalyst powder is accurately weighed and sealed in a test tube. The test tube was pre-dried at 90 ℃ for several hours with a vacuum dryer as a sample for measurement. An electronic balance (AW220) manufactured by Shimadzu corporation was used for weighing. In the case of the coated sheet, the net weight of the coated layer obtained by subtracting the weight of Teflon (registered trademark) (base material) of the same area from the total weight of the coated sheet was used as the sample weight of about 0.03 to 0.04 g. Next, the BET specific surface area was measured under the following measurement conditions. A BET curve is prepared on the adsorption side of the adsorption/desorption isotherm in the range of about 0.00 to 0.45 from the relative pressure (P/P0), and the BET specific surface area is calculated from the inclination and the slice.

[ solution 1]

< measurement Condition >

A measuring device: BELSORP36, a high-precision full-automatic gas adsorption device manufactured by Japan ベル K.K

adsorbing gas: n2

Dead volume measurement gas: he (He)

Adsorption temperature: 77K (liquid nitrogen temperature)

Pretreatment in measurement: vacuum drying at 90 deg.C for several hours (after He has been removed, place on the measurement bench)

measurement mode: isothermal adsorption and desorption processes

Measuring relative pressure (P/P0): about 0 to 0.99

Balance setting time: 180sec per 1 relative pressure

The method for producing the catalyst having the above specific surface area is not particularly limited, but the method described in Japanese patent application laid-open No. 2010-208887 or International publication No. 2009/075264 is preferably used.

the material of the carrier is not particularly limited as long as it is a material having a sufficient specific surface area and sufficient electron conductivity to allow the catalyst component to be supported in a dispersed state in the inside of the fine pores. Preferably the major component is carbon. Specifically, carbon particles composed of carbon black (ketjen black, oil furnace black, channel black, lampblack black, thermal black, acetylene black, etc.), activated carbon, and the like can be mentioned. "the main component is carbon" means that it contains carbon atoms as a main component, and is a concept including both of carbon atoms alone and carbon atoms substantially, and may contain elements other than carbon atoms. "consisting essentially of carbon atoms" means that impurities of about 2 to 3 wt% or less can be allowed to be mixed.

Carbon Black is more preferably used because a desired void region is easily formed inside the carrier, and Black Pearls (registered trademark) is particularly preferably used.

Further, it is preferable to control the crystallinity of the carbon support for the purpose of improving the corrosion resistance of the catalyst layer. For the crystallinity and crystalline composition of the carbon material, for example, G-band peak intensity and D-band peak intensity calculated by raman scattering spectral analysis can be used.

In addition to the carbon material, porous metal such as Sn (tin) or Ti (titanium), and further metal oxide having conductivity, for example, RuO2 or TiO2, can be preferably used as a support. By using such a metal oxide, corrosion of the carrier is reduced, and the durability of the catalyst is further improved.

The BET specific surface area of the carrier may be a specific surface area sufficient for supporting the catalyst component in a highly dispersed state. The BET specific surface area of the carrier is substantially equal to that of the catalyst. The BET specific surface area of the carrier is preferably 800m2/g or more, more preferably 1400m2/g or more. Since sufficient fine pores can be secured by the specific surface area as described above, a large amount of the metal catalyst can be stored (supported) in the fine pores. Therefore, the covering of the metal catalyst with the electrolyte in the catalyst layer can be suppressed (the contact between the metal catalyst and the electrolyte can be more effectively suppressed and prevented). Therefore, the activity of the metal catalyst can be more effectively utilized to further effectively promote the catalytic reaction.

The average particle diameter of the carrier is preferably 20 to 100 nm. If the pore structure is within this range, the thickness of the catalyst layer can be controlled to an appropriate range while maintaining the mechanical strength even when the above-described pore structure is provided on the support. As the value of the "average particle diameter of the carrier", unless otherwise mentioned, a value calculated as an average value of particle diameters of particles observed in a field of several to several tens using an observation device such as a Scanning Electron Microscope (SEM) or a Transmission Electron Microscope (TEM) is used. The "particle diameter" refers to the maximum distance among the distances between any two points on the contour line of the particle.

In the present invention, it is not necessary to use the above-mentioned particulate porous carrier as long as it has the above-mentioned specific surface area in the catalyst.

That is, examples of the carrier include a non-woven fabric, carbon paper, carbon cloth, and the like made of carbon fibers, which constitute a non-porous conductive carrier or a gas diffusion layer. In this case, the catalyst may be supported on the non-porous conductive carrier, or may be directly attached to a nonwoven fabric, carbon paper, carbon cloth, or the like made of carbon fibers, which constitutes a gas diffusion layer of the membrane electrode assembly.

The metal catalyst usable in the present invention has a function of playing a catalytic role of an electrochemical reaction. The metal catalyst used in the anode catalyst layer is not particularly limited as long as it has a catalytic action on the oxidation reaction of hydrogen, and a known catalyst can be used in the same manner. The metal catalyst used in the cathode catalyst layer is not particularly limited as long as it has a catalytic action on the reduction reaction of oxygen, and a known catalyst can be used in the same manner. Specifically, the metal may be selected from metals such as platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, copper, silver, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, and alloys thereof.

Among them, a catalyst containing at least platinum is preferably used in order to improve catalytic activity, resistance to poisoning by carbon monoxide and the like, heat resistance and the like. That is, the metal catalyst is preferably platinum, or preferably contains platinum and a metal component other than platinum, and more preferably platinum or a platinum-containing alloy. Such a metal catalyst can exert high activity. The composition of the alloy also depends on the type of metal to be alloyed, but the content of platinum may be 30 to 90 atomic%, and the content of metal to be alloyed with platinum may be 10 to 70 atomic%. The alloy is generally an alloy in which one or more metal elements or nonmetal elements are added to a metal element, and is a generic term for an alloy having a metallic property. In the structure of the alloy, it can be said that the alloy has a eutectic alloy in which the constituent elements are individually crystals, a substance in which the constituent elements are completely dissolved to form a solid solution, a substance in which the constituent elements form an intermetallic compound or a compound of a metal and a nonmetal, and the like, and any substance may be used in the present application. In this case, the metal catalyst used in the anode catalyst layer and the metal catalyst used in the cathode catalyst layer may be appropriately selected from those described above. In the present specification, unless otherwise specified, the description of the metal catalyst for the anode catalyst layer and the metal catalyst for the cathode catalyst layer are defined similarly for both. However, the metal catalysts of the anode catalyst layer and the cathode catalyst layer do not need to be the same, and may be appropriately selected so as to exhibit the desired functions as described above.

The shape and size of the metal catalyst (catalyst component) are not particularly limited, and the same shape and size as those of known catalyst components can be used. The shape may be, for example, granular, scaly, or lamellar, but is preferably granular. In this case, the average particle diameter of the metal catalyst (metal catalyst particle) is not particularly limited, but is preferably 3nm or more, more preferably more than 3nm and 30nm or less, and particularly preferably more than 3nm and 10nm or less. When the average particle diameter of the metal catalyst is 3nm or more, the metal catalyst is relatively firmly supported in the fine pores, and contact with the electrolyte in the catalyst layer is more effectively suppressed and prevented. In addition, elution due to potential change can be prevented, and deterioration of performance over time can be suppressed. Therefore, the catalytic activity can be further improved, that is, the catalytic reaction can be more effectively promoted. On the other hand, if the average particle diameter of the metal catalyst particles is 30nm or less, the metal catalyst can be supported in the fine pores of the carrier by a simple method, and the electrolyte coverage of the metal catalyst can be reduced. The "average particle diameter of the metal catalyst particles" in the present invention can be measured as a crystal grain diameter determined from the half width of the diffraction peak of the metal catalyst component by X-ray diffraction or an average value of the particle diameters of the metal catalyst particles examined by a Transmission Electron Microscope (TEM). In the present specification, the "average particle diameter of the metal catalyst particles" is an average value of particle diameters of the metal catalyst particles examined by a transmission electron microscope with respect to a statistically significant number (for example, at least 203) of samples.

In this embodiment, the catalyst content (mg/cm2) per unit catalyst-coated area is not particularly limited as long as sufficient dispersibility of the catalyst on the carrier and power generation performance can be obtained, and is, for example, 0.01 to 1mg/cm 2. In the case where the catalyst contains platinum or a platinum-containing alloy, the platinum content per unit catalyst-coated area is preferably 0.5mg/cm2 or less. The use of expensive noble metal catalysts, represented by platinum (Pt) or platinum alloys, is a major cause of the high price of fuel cells. Therefore, it is preferable to reduce the amount of expensive platinum (platinum content) to the above range to reduce the cost. The lower limit is not particularly limited as long as the power generation performance can be obtained, and is, for example, 0.01mg/cm2 or more. More preferably, the platinum content is 0.02 to 0.4mg/cm 2. In this embodiment, the activity per weight of the catalyst can be improved by controlling the pore structure of the carrier, and therefore the amount of expensive catalyst used can be reduced.

In addition, in the present specification, as the measurement (confirmation) of "the catalyst (platinum) content per unit catalyst coated area (mg/cm 2)", inductively coupled plasma emission spectroscopy (ICP) was used. The method of capturing the desired "content of catalyst (platinum) per unit catalyst coating area (mg/cm 2)" can be easily performed by those skilled in the art, and the amount can be adjusted by controlling the composition (catalyst concentration) and the coating amount of the slurry.

The amount of the catalyst supported on the carrier (also referred to as a loading rate in some cases) may be preferably 50 wt% or less, and more preferably 30 wt% or less, based on the total amount of the catalyst carrier (i.e., the carrier and the catalyst). When the amount of the catalyst component to be supported is within the above range, the catalyst component can be dispersed on the carrier sufficiently, the power generation performance can be improved, economic advantages can be obtained, and the catalyst activity per unit weight can be obtained. In addition, the amount of water present on the surface of the metal catalyst is preferably reduced. The lower limit of the amount of the carrier is not particularly limited, but is preferably 5% by weight or more.

The acidic group of the catalyst of the present invention is not particularly limited, and is preferably at least one selected from the group consisting of a hydroxyl group, a lactone group and a carboxyl group. If the acid group is any of these groups, the above-mentioned effects can be obtained more effectively.

The amount of the acidic group contained in the catalyst was 0.75mmol/g or less per the support. When the amount of water exceeds 0.75mmol/g or less, the amount of water existing in the vicinity of the metal catalyst becomes too large, and the oxygen reduction reaction activity is lowered, resulting in a decrease in the catalytic activity. The amount of the acidic group is preferably less than 0.7mmol/g of carrier, and more preferably 0.6mmol/g or less of carrier. More preferably 0.4mmol/g or less of the carrier. The lower limit of the amount of the acidic group is not particularly limited, but is preferably 0.1mmol/g support or more, and more preferably 0.2mmol/g support or more.

The amount of the acidic groups is measured by titration using a basic compound. However, specifically, the measurement was carried out by the method described in examples.

The method for adding the acidic group to the catalyst is not particularly limited, and for example, a wet method of impregnating a carrier (catalyst support) on which a metal catalyst is supported in an oxidizing solution containing an oxidizing agent or a heat treatment of the catalyst support can be employed. Details of this heat treatment will be described later.

[ catalyst layer ]

As described above, the catalyst of the present invention can exert high catalytic activity, that is, can promote a catalytic reaction. Therefore, the catalyst of the present invention can be preferably used for an electrode catalyst layer for a fuel cell. That is, the present invention also provides an electrode catalyst layer for a fuel cell containing the catalyst and the electrode catalyst of the present invention.

as shown in fig. 2, in the catalyst layer of the present invention, the catalyst is covered with the electrolyte 26, but the electrolyte 26 does not intrude into the cavity 24 of the catalyst (carrier 23). Therefore, the metal catalyst 22 on the surface of the carrier 23 is in contact with the electrolyte 26, but the metal catalyst 22 supported in the cavity 24 is not in contact with the electrolyte 26. The metal catalyst in the cavity forms a three-phase interface of oxygen and water in a non-contact state with the electrolyte, and thereby the reactive area of the metal catalyst can be ensured.

The catalyst of the present invention may be present in either the cathode catalyst layer or the anode catalyst layer, but is preferably used in the cathode catalyst layer. This is because, as described above, the catalyst of the present invention can effectively utilize the catalyst by forming a three-phase interface with water even without contacting the electrolyte, but water is formed in the cathode catalyst layer.

The electrolyte is not particularly limited, but is preferably an ion-conductive polymer electrolyte. The polymer electrolyte is also called a proton conductive polymer because it functions to transfer protons generated around the catalyst active material on the fuel electrode side.

the polymer electrolyte is not particularly limited, and conventionally known knowledge can be appropriately referred to. The polymer electrolyte is roughly classified into a fluorine polymer electrolyte and a hydrocarbon polymer electrolyte according to the kind of the ion exchange resin as a constituent material.

Examples of the ion exchange resin constituting the fluorine-based polymer electrolyte include: perfluorosulfonic acid-based polymers such as perfluorosulfonic acid resin (Nafion) (registered trademark, manufactured by Dupont), perfluorocarboxylic acid resin (registered trademark, manufactured by asahi chemical company, limited), perfluorocarbon carbonic acid resin (registered trademark, manufactured by asahi nitroxide corporation), perfluorocarbon phosphonic acid-based polymers, trifluorostyrene sulfonic acid-based polymers, ethylene tetrafluoroethylene-g-styrene sulfonic acid-based polymers, ethylene-tetrafluoroethylene copolymers, polyvinylidene fluoride-perfluorocarbon sulfonic acid-based polymers, and the like. These fluorine-based polyelectrolytes are preferably used from the viewpoint of excellent heat resistance, chemical stability, durability, and mechanical strength, and particularly, a fluorine-based polyelectrolyte composed of a perfluorocarbon sulfonic acid-based polymer is preferably used.

Specific examples of the hydrocarbon electrolyte include: sulfonated polyether sulfone (S-PES), sulfonated polyaryletherketone, sulfonated poly (p-benzimidazolyl) alkyl, phosphonated poly (p-benzimidazolyl) alkyl, sulfonated poly (p-styrene), sulfonated polyether ether ketone (S-PEEK), sulfonated poly (p-phenylene terephthalate) (S-PPP), and the like. These hydrocarbon-based polymer electrolytes are preferably used from the viewpoint of production that the raw materials are inexpensive and the production process is simple, and the selectivity of the material is high. The ion exchange resin may be used alone or in combination of two or more. In addition, other materials may be used without being limited to the above materials.

in the polymer electrolyte that is responsible for proton transfer, the degree of proton conductivity is important. Here, when the EW of the polymer electrolyte is too large, the ion conductivity of the entire catalyst layer is lowered. Therefore, the catalyst layer of the present embodiment preferably contains a polymer electrolyte having a small EW. Specifically, the catalyst layer of the present embodiment preferably contains a polymer electrolyte having an EW of 1500g/eq or less, more preferably 1200g/eq or less, and particularly preferably 1000g/eq or less.

On the other hand, if the EW is too small, the hydrophilicity is too high, and smooth movement of water is difficult. From this viewpoint, the EW of the polymer electrolyte is preferably 500 or more. In addition, ew (equivalent weight) represents the equivalent weight of the exchange group having proton conductivity. The equivalent weight is the dry weight of the ion-exchange membrane per 1 equivalent of ion-exchange groups, expressed in "g/eq" units.

In addition, the catalyst layer contains two or more types of polymer electrolytes having different EW in the power generation plane, and in this case, the polymer electrolyte having the lowest EW among the polymer electrolytes is preferably used in a region where the relative humidity of the gas in the flow path is 90% or less. By adopting such a material configuration, the resistance value becomes small regardless of whether it is a current density region, and the battery performance can be improved. The polyelectrolyte used in the region where the relative humidity of the gas in the flow path is 90% or less, that is, the polyelectrolyte having the lowest EW, preferably has an EW of 900g/eq or less. This makes the above-described effect more reliable and remarkable.

further, it is preferable to use the polymer electrolyte having the lowest EW in a region having a temperature higher than the average temperature of the inlet and outlet of the cooling water. With this, the resistance value becomes small regardless of whether the current density region is present, and the battery performance can be further improved.

Further, from the viewpoint of reducing the resistance value of the fuel cell system, the polymer electrolyte having the lowest EW is preferably used in a region within a range of 3/5 or less from the gas supply port of at least one of the fuel gas and the oxidizing gas with respect to the flow path length.

The catalyst layer of the present embodiment may contain a liquid proton conductive material between the catalyst and the polymer electrolyte, the liquid proton conductive material being capable of connecting the catalyst and the polymer electrolyte in a proton conductive state. By introducing the liquid proton conductive material, a proton transport path through the liquid proton conductive material can be ensured between the catalyst and the polymer electrolyte, and protons necessary for power generation can be efficiently transported to the catalyst surface. This improves the utilization efficiency of the catalyst, and therefore, the amount of the catalyst used can be reduced while maintaining the power generation performance. The liquid proton conductive material may be sandwiched between the catalyst and the polymer electrolyte, and may be disposed in the pores (secondary pores) between the porous carriers or the pores (micropores or micropores: primary pores) in the porous carriers in the catalyst layer.

The liquid proton conductive material is not particularly limited as long as it has ion conductivity and functions to form a proton transport path between the catalyst and the polymer electrolyte. Specifically, there may be mentioned: water, a protic ionic liquid, a perchloric acid aqueous solution, a nitric acid aqueous solution, a formic acid aqueous solution, an acetic acid aqueous solution, and the like.

In the case of using water as the liquid proton conductive material, by wetting the catalyst layer with a small amount of liquid water or humidified gas before starting power generation, water as the liquid proton conductive material can be introduced into the catalyst layer. In addition, the generated water produced by the electrochemical reaction during the operation of the fuel cell may also be utilized as the liquid proton conductive material. Therefore, it is not necessary to hold the liquid proton conductive material in the state where the operation of the fuel cell is started. For example, the surface distance between the catalyst and the electrolyte is preferably 0.28nm or more, which is the diameter of oxygen ions constituting water molecules. By maintaining such a distance, water (liquid proton conductive material) can be interposed between the catalyst and the polymer electrolyte (liquid conductive material holding portion) while maintaining a non-contact state between the catalyst and the polymer electrolyte, and a proton transport path realized by water between the catalyst and the polymer electrolyte can be ensured.

When a liquid other than water, such as an ionic liquid, is used as the liquid proton conductive material, it is preferable to disperse the ionic liquid, the polymer electrolyte, and the catalyst in a solution when the catalyst ink is produced.

In the catalyst of the present invention, the total area of the catalyst in contact with the polymer electrolyte is smaller than the total area of the catalyst exposed to the liquid conductive material holding portion.

the comparison of the two areas can be performed by, for example, obtaining the magnitude relation of the capacities of the electric double layers formed at the catalyst-polyelectrolyte interface and the catalyst-liquid proton conductive material interface in a state where the liquid proton conductive material holding portion is filled with the liquid proton conductive material. That is, since the electric double layer capacity is proportional to the area of the electrochemically effective interface, if the electric double layer capacity formed at the catalyst-electrolyte interface is smaller than the electric double layer capacity formed at the catalyst-liquid proton conductive material interface, the contact area of the catalyst with the electrolyte is smaller than the exposed area of the liquid conductive material holding portion.

Here, a description will be given of a method of measuring the electric double layer capacity formed at each of the catalyst-electrolyte interface and the catalyst-liquid proton conductive material interface, that is, a magnitude relation of the contact area between the catalyst and the electrolyte and between the catalyst and the liquid proton conductive material (a method of determining a magnitude relation between the contact area of the catalyst with the electrolyte and the exposed area of the catalyst to the liquid conductive material holding portion).

That is, in the catalyst layer of the present embodiment,

(1) Catalyst-polyelectrolyte (C-S)

(2) Catalyst-liquid proton conducting Material (C-L)

(3) Porous carrier-polyelectrolyte (Cr-S)

(4) Porous carrier-liquid proton conductive material (Cr-L)

these four interfaces above may contribute to the capacity as a double electric layer (Cdl).

Since the electric double layer capacity is proportional to the area of the electrochemically effective interface as described above, CdlC-S (electric double layer capacity at the catalyst-polymer electrolyte interface) and CdlC-L (electric double layer capacity at the catalyst-liquid proton conductive material interface) may be determined. The contributions of the four interfaces to the electric double layer capacity (Cdl) can be separated as follows.

First, the electric double layer capacity is measured under high humidification conditions such as 100% RH and low humidification conditions such as 10% RH or less, for example. Further, as a method for measuring the electric double layer capacity, cyclic voltammetry, electrochemical impedance spectroscopy, or the like can be given. From the comparison of the two, it is possible to separate the contribution of the liquid proton conductive material (in this case, "water"), that is, the above-described (2) and (4).

By further deactivating the catalyst, for example, in the case of using Pt as the catalyst, the catalyst can be deactivated by supplying CO gas to the electrode to be measured and adsorbing CO on the surface of Pt, thereby making it possible to separate the contribution to the electric double layer capacity. In this state, the electric double layer capacities under the high humidification condition and the low humidification condition are measured in the same manner as described above, and from the comparison of the two, the contribution of the catalyst, that is, (1) and (2) above can be separated.

As described above, all the contributions of (1) to (4) above can be separated, and the electric double layer capacity formed at the interface between the catalyst, the polymer electrolyte, and the liquid proton conductive material can be determined.

That is, the measured value (a) in the high humidified state is the electric double layer capacity formed at the entire interface of the above (1) to (4), and the measured value (B) in the low humidified state is the electric double layer capacity formed at the interface of the above (1) and (3). The measured value (C) in the catalyst-deactivated, high-humidified state is the electric double layer capacity formed at the interface between (3) and (4), and the measured value (D) in the catalyst-deactivated, low-humidified state is the electric double layer capacity formed at the interface between (3).

Therefore, the difference between a and C is the electric double layer capacity formed at the interface of (1) and (2), and the difference between B and D is the electric double layer capacity formed at the interface of (1). Then, if the difference between these values, (A-C) - (B-D) is calculated, the electric double layer capacity formed at the interface of (2) can be obtained. The contact area of the catalyst with the polymer electrolyte or the exposed area to the conductive material holding portion may be determined by TEM (transmission electron microscope) tomography or the like, for example, in addition to the above-described method.

The coverage of the electrolyte with respect to the metal catalyst is 0.5 or less, preferably 0.4 or less, and more preferably 0.23 or less (lower limit value is 0). When the coverage exceeds 0.5, the oxygen reduction reaction activity is lowered and the catalytic activity is lowered.

The coverage of the electrolyte can be calculated from the electric double layer capacity, specifically, the method described in the examples.

The catalyst layer contains, as required: water-proofing agents such as polytetrafluoroethylene, polyhexafluoropropylene and tetrafluoroethylene-hexafluoropropylene copolymers, dispersing agents such as surfactants, thickeners such as glycerin, Ethylene Glycol (EG), polyvinyl alcohol (PVA) and Propylene Glycol (PG), and additives such as pore-forming agents may be used.

The thickness (dry film thickness) of the catalyst layer is preferably 0.05 to 30 μm, more preferably 1 to 20 μm, and still more preferably 2 to 15 μm. The above thickness is applied to both the cathode catalyst layer and the anode catalyst layer. However, the thicknesses of the cathode catalyst layer and the anode catalyst layer may be the same or different.

(method for producing catalyst layer)

In the following, preferred embodiments for producing the catalyst layer are described, but the technical scope of the present invention is not limited to the following embodiments. Since various conditions such as the material of each constituent element of the catalyst layer are as described above, the description thereof is omitted here.

First, a support (also referred to as "porous support" or "conductive porous support" in the present specification) is prepared, and a hole structure is controlled by performing heat treatment on the support. Specifically, the carrier may be produced as described above. Thus, a carrier having a specific surface area can be obtained.

The conditions of the heat treatment vary depending on the material, and are appropriately determined so as to obtain a desired specific surface area. Such heat treatment conditions may be determined by the material while confirming the void structure, and can be easily determined by those skilled in the art. In addition, a technique of graphitizing a support by heat treatment at a high temperature has been known in the past. However, in the conventional heat treatment, the holes in the carrier are almost all blocked, and the control of the hole structure of the fine holes (expansion into shallow primary holes) near the catalyst cannot be performed.

Next, the catalyst is supported on the porous carrier to prepare a catalyst powder. The catalyst can be supported on the porous carrier by a known method. For example, the following may be used: known methods such as impregnation, liquid phase reduction supporting, evaporation drying, colloid adsorption, spray pyrolysis, and reverse micelle (microemulsion).

Next, the obtained catalyst powder is subjected to heat treatment in a hydrogen atmosphere to reduce acidic groups. The temperature of the heat treatment is preferably 200 to 1400 ℃, and the time of the heat treatment is preferably 1 to 10 hours.

Next, a catalyst ink containing a catalyst powder to which an acidic group is added, a polymer electrolyte, and a solvent is prepared. The solvent is not particularly limited, and a common solvent used for forming the catalyst layer can be used. Specifically, water such as tap water, purified water, ion-exchanged water, and distilled water, lower alcohols having 1 to 4 carbon atoms such as cyclohexanol, methanol, ethanol, n-propanol (n-Propyl alcohol), isopropanol, n-butanol, sec-butanol, isobutanol, and tert-butanol, propylene glycol, benzene, toluene, and xylene, and the like can be mentioned. In addition to these, butanol acetate, dimethyl ether, ethylene glycol, or the like may be used as the solvent. These solvents may be used alone or in the form of a mixture of two or more kinds.

The amount of the solvent constituting the catalyst ink is not particularly limited as long as it is an amount that can completely dissolve the electrolyte. Specifically, the concentration of the solid component obtained by adding the catalyst powder and the polymer electrolyte together is preferably about 1 to 50 wt%, more preferably about 5 to 30 wt% in the electrode catalyst ink.

When additives such as a water repellent agent, a dispersant, a thickener, and a pore former are used, these additives may be added to the catalyst ink. In this case, the amount of the additive to be added is not particularly limited as long as it is an amount that does not impair the above effects of the present invention. For example, the additive is preferably added in an amount of 5 to 20 wt% based on the total weight of the electrode catalyst ink.

Next, a catalyst ink is applied to the surface of the substrate. The method of coating the substrate is not particularly limited, and a known method can be used. Specifically, the coating can be carried out by a known method such as a spray (spray coating) method, an ink printing method, a die coating method, a screen printing method, or a blade coating method.

At this time, as a substrate to which the catalyst ink is applied, a solid polymer electrolyte membrane (electrolyte layer) or a gas diffusion substrate (gas diffusion layer) may be used. In this case, the catalyst layer is formed on the surface of the solid polymer electrolyte membrane (electrolyte layer) or the gas diffusion substrate (gas diffusion layer), and the resulting laminate is used as it is for producing a membrane electrode assembly. Alternatively, a releasable substrate such as a Polytetrafluoroethylene (PTFE) [ Teflon (registered trademark) ] sheet may be used as the substrate, and the catalyst layer may be obtained by forming the catalyst layer on the substrate and then partially peeling the catalyst layer from the substrate.

Finally, the coating layer (film) of the catalyst ink is dried for 1 to 60 minutes at room temperature to 150 ℃ in an air atmosphere or an inert gas atmosphere. Thereby, a catalyst layer is formed.

(Membrane electrode Assembly)

According to a further embodiment of the present invention, there is provided a membrane electrode assembly for a fuel cell, comprising a solid polymer electrolyte membrane 2, a cathode catalyst layer disposed on one side of the electrolyte membrane, an anode catalyst layer disposed on the other side of the electrolyte membrane, and a pair of gas diffusion layers (4a, 4c) sandwiching the electrolyte membrane 2, the anode catalyst layer 3a, and the cathode catalyst layer 3 c. In this membrane electrode assembly, at least one of the cathode catalyst layer and the anode catalyst layer is the catalyst layer of the above-described embodiment.

Among them, in consideration of the necessity of improving proton conductivity and the transport property (gas diffusibility) of the reaction gas (particularly O2), at least the cathode catalyst layer is preferably the catalyst layer of the above-described embodiment. The catalyst layer in the above-described embodiment may be used as an anode catalyst layer, or may be used as both a cathode catalyst layer and an anode catalyst layer, and the like, and is not particularly limited.

According to a further embodiment of the present invention, there is provided a fuel cell having the membrane electrode assembly in the above-described aspect. That is, one embodiment of the present invention is a fuel cell including a pair of anode separators and cathode separators sandwiching the membrane electrode assembly in the above-described form.

Next, the constituent elements of the PEFC1 using the catalyst layer of the above embodiment will be described with reference to fig. 1. Among them, the present invention is characterized in that the catalyst and the catalyst layer are formed. Therefore, the specific form of the member other than the catalyst layer constituting the fuel cell can be changed as appropriate with reference to conventionally known knowledge.

(electrolyte Membrane)

As shown in fig. 1, the electrolyte membrane is made of, for example, a solid polymer electrolyte membrane 2. The solid polymer electrolyte membrane 2 has a function of selectively allowing protons generated in the anode catalyst layer 3a to permeate through the cathode catalyst layer 3c in the membrane thickness direction during the operation of the PEFC 1. The solid polymer electrolyte membrane 2 also functions as a partition wall for preventing mixing of the fuel gas supplied to the anode side and the oxidant gas supplied to the cathode side.

The electrolyte material constituting the solid polymer electrolyte membrane 2 is not particularly limited, and conventionally known findings can be appropriately referred to. For example, a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte described above as a polymer electrolyte can be used. In this case, it is not necessary to use the same polymer electrolyte as that used for the catalyst layer.

The thickness of the electrolyte layer is not particularly limited as long as it is appropriately determined in consideration of the characteristics of the resulting fuel cell. The thickness of the electrolyte layer is usually about 5 to 300 μm. When the thickness of the electrolyte layer is within such a range, the balance between the strength at the time of film formation, the durability at the time of use, and the output characteristics at the time of use can be appropriately controlled.

(gas diffusion layer)

The gas diffusion layers (anode gas diffusion layer 4a, cathode gas diffusion layer 4c) have a function of promoting diffusion of the catalyst layers (3a, 3c) to the gases (fuel gas or oxidant gas) supplied through the gas flow paths (6a, 6c) of the separators, and a function of serving as electron conduction paths.

The material of the base material constituting the gas diffusion layers (4a, 4c) is not particularly limited, and conventionally known knowledge can be appropriately referred to. Examples thereof include: sheet-like materials having electrical conductivity and porosity such as carbon woven fabrics, paper sheets, felts, and nonwoven fabrics. The thickness of the substrate may be determined appropriately in consideration of the properties of the gas diffusion layer to be obtained, and may be about 30 to 500 μm. If the thickness of the base material is a value within such a range, the balance between the mechanical strength and the diffusibility of gas, water, and the like can be appropriately controlled.

The gas diffusion layer preferably contains a water repellent agent for the purpose of further improving water repellency to prevent a flooding phenomenon or the like. The water repellent is not particularly limited, but includes: fluorine-based polymer materials such as Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyhexafluoropropylene, tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polypropylene, polyethylene, and the like.

In order to further improve the water repellency, the gas diffusion layer may have a carbon particle layer (microporous layer; MPL, not shown) composed of an aggregate of carbon particles containing a water repellent on the catalyst layer side of the substrate.

The carbon particles contained in the carbon particle layer are not particularly limited, and conventionally known materials such as carbon black, graphite, and expanded graphite can be suitably used. Among them, carbon black such as oil furnace carbon black, channel carbon black, lamp fume carbon black, thermal carbon black, and acetylene black is preferably used because of excellent electron conductivity and a large specific surface area. The average particle diameter of the carbon particles may be about 10 to 100 nm. This can provide high drainage by capillary force and also improve contact with the catalyst layer.

Examples of the water repellent agent used for the carbon particle layer include the same water repellent agents as described above. Among them, a fluorine-based polymer material is preferably used because of its excellent water resistance, corrosion resistance during electrode reaction, and the like.

In consideration of the balance between the water repellency and the electron conductivity, the mixing ratio of the carbon particles and the water repellent agent in the carbon particle layer may be set to 90: 10-40: 60 (carbon particles: water repellent agent). The thickness of the carbon particle layer is not particularly limited, and may be appropriately determined in consideration of the water resistance of the gas diffusion layer to be obtained.

(method of producing Membrane electrode Assembly)

The method for producing the membrane electrode assembly is not particularly limited, and conventionally known methods can be used. For example, the following may be used: a method of bonding a gas diffusion layer to a layer obtained by transferring or coating a catalyst layer onto a solid polymer electrolyte membrane by a hot press and drying the catalyst layer, or a method of manufacturing two Gas Diffusion Electrodes (GDEs) by pre-coating and drying a catalyst layer on a microporous layer side (one side of a base layer in the case where a microporous layer is not included) of a gas diffusion layer and then bonding the gas diffusion electrodes to both sides of a solid polymer electrolyte membrane by a hot press. The conditions for coating and bonding by a hot press may be appropriately adjusted depending on the type of the polymer electrolyte (perfluorosulfonic acid-based or hydrocarbon-based) in the solid polymer electrolyte membrane or the catalyst layer.

(baffle)

When a plurality of unit cells of a fuel cell such as a polymer electrolyte fuel cell are connected in series to form a fuel cell stack, the separator has a function of electrically connecting the unit cells in series. The separator also functions as a partition wall for separating the fuel gas, the oxidizing gas, and the coolant from each other. In order to secure these flow paths, it is preferable to provide the gas flow paths and the cooling flow paths in the separators, respectively, as described above. As the material constituting the separator, conventionally known materials such as carbon such as dense carbon graphite and carbon plate, and metals such as stainless steel can be used without limitation. The thickness and size of the separator, the shape and size of each flow channel provided, and the like are not particularly limited, and may be appropriately determined in consideration of the desired output characteristics and the like of the fuel cell to be obtained.

The method for producing the fuel cell is not particularly limited, and conventionally known knowledge is appropriately referred to in the field of fuel cells.

further, a fuel cell stack having a structure in which a plurality of membrane electrode assemblies are stacked and connected in series via separators may be formed so that a fuel cell can exhibit a desired voltage. The shape and the like of the fuel cell are not particularly limited, and may be appropriately determined so that desired cell characteristics such as voltage can be obtained.

The PEFC or the membrane electrode assembly described above uses a catalyst layer having excellent power generation performance and durability. Therefore, the PEFC or the membrane electrode assembly is excellent in power generation performance and durability.

The PEFC and the fuel cell stack using the PEFC according to the present embodiment can be mounted on a vehicle as a driving power source, for example.

Examples

The effects of the present invention will be described with reference to the following examples and comparative examples. Here, the technical scope of the present invention is not limited to the following examples.

(example 1)

Black pearls (registered trademark) having a BET specific surface area of 1440m2/g was prepared (support A).

A catalyst powder A was obtained by using a metal catalyst comprising the above carrier A and platinum (Pt) having an average particle diameter of 4nm supported on the carrier A so that the supporting ratio was 50% by weight. That is, 46g of the carrier A was immersed in 1000g (platinum content: 46g) of dinitrodiamine platinum nitric acid solution having a platinum concentration of 4.6 mass%, and stirred, and then 100ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours to support platinum on the carrier a. Then, the catalyst powder a having a loading rate of 50% by weight was obtained by filtration and drying. Thereafter, the reaction mixture was maintained at 900 ℃ for 1 hour in a hydrogen atmosphere to obtain catalyst powder A.

with respect to the thus-obtained catalyst powder A, the BET specific surface area was measured and found to be 1291m2/g of carrier.

The catalyst powder a having an acidic group and an ionomer dispersion (Nafion (registered trademark) D2020, EW 1100g/mol, manufactured by DuPont) as a polymer electrolyte were mixed so that the weight ratio of the carbon support to the ionomer was 0.9. Further, an n-propanol solution (50%) was added as a solvent so that the solid content fraction (Pt + carbon support + ionomer) became 7 wt%, to prepare a cathode catalyst ink.

Ketjen black (particle size: 30 to 60nm) was used as a carrier, and platinum (Pt) having an average particle size of 2.5nm was supported on the carrier as a metal catalyst so that the supporting rate became 50 wt%, to obtain a catalyst powder. This catalyst powder and an ionomer dispersion (Nafion (registered trademark) D2020, EW 1100g/mol, manufactured by DuPont) as a polymer electrolyte were mixed so that the weight ratio of the carbon support and the ionomer became 0.9. Further, an n-propanol solution (50%) was added as a solvent so that the solid content fraction (Pt + carbon support + ionomer) became 7%, to prepare an anode catalyst ink.

Then, gaskets (polynaphthalene ester (registered trademark) manufactured by Dupont, thickness: 25 μm (adhesive layer: 10 μm)) were disposed around both surfaces of the polymer electrolyte membrane (Nafion (registered trademark) NR211, manufactured by Dupont, thickness: 25 μm). Next, a catalyst ink was applied to the exposed portion of one surface of the polymer electrolyte membrane by a spray coating method, and the catalyst ink was applied to a size of 5cm × 2 cm. The catalyst ink was dried by holding the table subjected to spray coating at 60 ℃ for 1 minute, to obtain a cathode catalyst layer. The amount of platinum carried in this case was 0.35mg/cm 2. Subsequently, similarly to the cathode catalyst layer, the electrolyte membrane was subjected to spray coating and heat treatment to form an anode catalyst layer, thereby obtaining a membrane electrode assembly (1) (MEA (1)) of the present example.

(example 2)

A carrier B having a BET specific surface area of 1720m2/g was prepared. Specifically, the carrier B is prepared by the method described in international publication No. 2009/075264, etc.

A catalyst powder B was obtained by using a metal catalyst comprising the above-mentioned carrier B and platinum (Pt) having an average particle diameter of 4nm supported on the carrier B so that the supporting rate became 30% by weight. That is, 46g of the carrier A was immersed in 1000g (platinum content: 46g) of dinitrodiamine platinum nitric acid solution having a platinum concentration of 4.6 mass%, and stirred, and then 100ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours to support platinum on the carrier B. Then, the catalyst powder B was filtered and dried to obtain a catalyst powder B having a supporting ratio of 30 wt%. Thereafter, the mixture was kept at 900 ℃ for 1 hour in a hydrogen atmosphere to obtain catalyst powder B.

With respect to the thus-obtained catalyst powder B, the BET specific surface area was measured and found to be 1753m2/g of carrier.

A membrane electrode assembly (2) (MEA (2)) was obtained in the same manner as in example 1, except that the catalyst powder B obtained in the above-described manner was used in place of the catalyst powder A, and the amount of platinum supported was 0.35mg/cm 2.

Comparative example 1

Ketjen black EC300J (manufactured by Ketjen Black International Inc.) having a BET specific surface area of 720m2/g (support C) was prepared.

The catalyst powder C was obtained by using a metal catalyst in which platinum (Pt) having an average particle diameter of 5nm was supported on the carrier C so that the supporting ratio became 50 wt%. That is, 46g of the carrier A was immersed in 1000g (platinum content: 46g) of dinitrodiamine platinum nitric acid solution having a platinum concentration of 4.6 mass%, and stirred, and then 100ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours to support platinum on the carrier C. Then, the mixture was filtered and dried to obtain catalyst powder C having a loading rate of 50 wt%.

For the catalyst powder C, an oxidizing solution treatment for adding an acidic group was performed. The catalyst powder C was immersed in a 3.0mol/L aqueous nitric acid solution at 80 ℃ for 2 hours. Then, the resultant was filtered and dried to obtain catalyst powder C having an acidic group.

With respect to the thus-obtained catalyst powder C, the BET specific surface area was measured and found to be 703m2/g of carrier.

A comparative membrane electrode assembly (1) (comparative MEA (1)) was obtained in the same manner as in example 1 except that the catalyst powder C obtained in the above was used instead of the catalyst powder a.

Comparative example 2

Ketjen black EC300J (manufactured by Ketjen Black International Inc.) having a BET specific surface area of 720m2/g (support D) was prepared.

The catalyst powder D was obtained by using a metal catalyst in which platinum (Pt) having an average particle diameter of 5nm was supported on the carrier D so that the supporting ratio was 50 wt%. That is, 46g of the carrier D was immersed in 1000g (platinum content: 46g) of a dinitrodiamine platinum nitric acid solution having a platinum concentration of 4.6 mass%, and stirred, and then 100ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours to support platinum on the carrier D. Then, the resultant was filtered and dried to obtain catalyst powder D having a supporting ratio of 50 wt%.

then, it was kept at 900 ℃ for 1 hour under a hydrogen atmosphere to obtain catalyst powder D. With respect to the thus-obtained catalyst powder D, the BET specific surface area was measured and found to be 711m2/g of carrier.

A comparative membrane electrode assembly (2) (comparative MEA (2)) was obtained in the same manner as in example 1 except that the catalyst powder D obtained above was used instead of the catalyst powder a.

Comparative example 3

Black pearls (registered trademark) having a BET specific surface area of 1440m2/g was prepared (support E).

A catalyst powder E was obtained by using a metal catalyst comprising the above-mentioned carrier E and platinum (Pt) having an average particle diameter of 4nm supported thereon so that the supporting ratio of the carrier E became 50% by weight. That is, 46g of the carrier E was immersed in 1000g (platinum content: 46g) of a dinitrodiamine platinum nitric acid solution having a platinum concentration of 4.6 mass%, and stirred, and then 100ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours to support platinum on the carrier E. Then, the resultant was filtered and dried to obtain catalyst powder E having a supporting ratio of 50 wt%.

For the catalyst powder E, an oxidizing solution treatment for adding an acidic group was performed. The catalyst powder C was immersed in a 3.0mol/L aqueous nitric acid solution at 80 ℃ for 3 hours. Then, the resultant was filtered and dried to obtain catalyst powder E having an acidic group.

With respect to the thus-obtained catalyst powder E, the BET specific surface area was measured and found to be 1236m2/g of carrier.

A comparative membrane electrode assembly (3) (comparative MEA (3)) was obtained in the same manner as in example 1 except that the catalyst powder E obtained in the above was used instead of the catalyst powder a.

Comparative example 4

A carrier F having a BET specific surface area of 1440m2/g was prepared. Specifically, the carrier F is prepared by the method described in international publication No. 2009/075264, etc.

Using the carrier F prepared as described above, platinum (Pt) having an average particle diameter of 4nm was supported on the carrier F as a metal catalyst so that the supporting ratio became 30 wt%, to obtain a catalyst powder F. That is, 138g of the carrier F was immersed in 1000g (platinum content: 46g) of dinitrodiamine platinum nitric acid solution having a platinum concentration of 4.6 mass% and stirred, and then 100ml of 100% ethanol was added as a reducing agent. This solution was stirred and mixed at the boiling point for 7 hours to support platinum on the carrier F. Then, the catalyst powder F was filtered and dried to obtain a catalyst powder F having a supporting ratio of 30 wt%.

For the catalyst powder F, an oxidizing solution treatment for adding an acidic group was performed. The catalyst powder C was immersed in a 3.0mol/L aqueous nitric acid solution at 80 ℃ for 2 hours. Then, the resultant was filtered and dried to obtain an acidic group-containing catalyst powder F.

With respect to the thus-obtained catalyst powder F, the BET specific surface area was measured and found to be 1743m2/g carrier.

A comparative membrane electrode assembly (4) (comparative MEA (4)) was obtained in the same manner as in example 2 except that the catalyst powder F obtained in the above was used instead of the catalyst powder a.

(measurement of acid base quantity)

The amount of acidic groups was measured by the titration method as follows. That is, first, the catalyst powder having 2.5g of acid groups was washed with 1L of warm pure water and dried. After drying, the amount of carbon contained in the catalyst having an acid group was measured to be 0.25g, and mixed with 55ml of water and stirred for 10 minutes, and then ultrasonic dispersion was performed for two minutes. Then, the catalyst dispersion was moved to a clean ball box by nitrogen gas, and the nitrogen gas was bubbled for 10 minutes. Then, 0.1M aqueous salt solution was excessively charged into the catalyst dispersion, and the basic solution was neutralized and titrated with 0.1M hydrochloric acid, and the amount of functional groups was determined from the neutralization point. Here, three kinds of NaOH, Na2CO3, and NaHCO3 were used for the salt-based aqueous solution, and neutralization titration was performed for each. This is because the type of the functional group used for neutralizing each salt group differs, and in the case of NaOH, neutralization reaction is performed with a carboxyl group, a lactone group and a hydroxyl group, in the case of Na2CO3, neutralization reaction is performed with a carboxyl group and a lactone group, and in the case of NaHCO3, neutralization reaction is performed with a carboxyl group. The amount of acidic groups was calculated from the results of the types and amounts of three types of salt groups to be added in the titration and the amount of hydrochloric acid to be consumed. In addition, in the confirmation of the neutralization point, a pH tester was used, pH7.0 is the neutralization point in the case of NaOH, pH8.5 is the neutralization point in the case of Na2CO3, and pH4.5 is the neutralization point in the case of NaHCO 3. From this, the total amount of carboxyl groups, lactone groups and hydroxyl groups added to the catalyst was determined.

[ coverage of electrolyte ]

Coverage of the electrolyte with respect to the metal catalyst the coverage of the catalyst with respect to the solid proton conductive material was calculated using a measurement of the electric capacity of the electric double layer formed at the interface of the solid proton conductive material and the liquid proton conductive material of the catalyst. In addition, for the calculation of the coverage, the ratio of the electric double layer capacity in the low humidified state to the electric double layer capacity in the high humidified state was calculated, and the measured values under the conditions of 5% RH and 100% RH were used as the values representing the humidified state.

(measurement of electric double layer Capacity)

The obtained MEA was measured for the electric double layer capacities in a high humidified state, a low humidified state, and further in a deactivated catalyst and high humidified state and low humidified state by electrochemical impedance spectroscopy, and the contact areas of the catalysts with the two proton conductive materials of the electrode catalysts of the two cells were compared.

Further, as the used equipment, a frequency response analyzer FRA5020 manufactured by electrical engineering corporation ブロック, an electrochemical measurement system HZ-3000 manufactured by beidou electrical corporation and エヌエフ circuit design, was used, and the measurement conditions shown in the following table 1 were adopted.

[ Table 1]

Temperature of single cell	30℃
		Frequency range	20kHz～10mHz
Amplitude of vibration	±10mV
		Holding potential	0.45V

Supplying gas (antipole/action pole)	H₂/N₂
		Humidity (antipode/action pole)	5/5％RH～100/100RH

First, each cell was heated to 30 ℃ by a heater, and the electric double layer capacity was measured in a state where nitrogen gas and hydrogen gas adjusted to the humidified state shown in table 1 were supplied to the working electrode and the counter electrode, respectively.

When measuring the electric double layer capacity, the potential of the working electrode was vibrated at a holding potential of 0.45V, an amplitude of. + -. 10mV, and a frequency range of 20kHz to 10mHz, as shown in Table 1.

That is, the real part and the imaginary part of the impedance at each frequency are obtained from the response of the electrode potential at the time of vibration. The relationship between the imaginary part (Z ") and the angular velocity ω (according to frequency conversion) is represented by the following equation (1). Therefore, the inverse number of the imaginary part is obtained for the-2 square of the angular velocity, and a value where the-2 square of the angular velocity is 0 is extrapolated, thereby obtaining the electric double layer capacity Cd 1.

Formula 1

Such measurements were carried out in the low humidified state and the high humidified state (condition of 5% RH → 10% RH → 90% RH → 100%) in this order.

Further, the electric double layer capacities in the high humidified state and the low humidified state in the deactivation of the Pt catalyst were measured similarly by allowing nitrogen gas containing CO at a concentration of 1% (volume ratio) to flow through the working electrode at 1 NL/min for 15 minutes or more. These results are shown in table 2. The obtained electric double layer capacity is converted into a value per unit area of the catalyst layer. In table 2, "-" means no measurement.

Further, based on the measured values, the electric double layer capacities formed at the catalyst-solid proton conductive material (C-S) interface and the catalyst-liquid proton conductive material (C-L) interface were calculated.

In this calculation, measured values of the conditions of 5% RH and 100% RH were used as values representing the electric double layer capacities in the low humidification state and the high humidification state, respectively.

Experiment 1: evaluation of oxygen reduction (ORR) Activity

The membrane electrode assemblies (1) to (2) prepared in examples 1 to 2 and the comparative membrane electrode assemblies (1) to (4) prepared in comparative examples 1 to 4 were evaluated for oxygen reduction (ORR) activity by measuring a generated current (μ a/cm2(Pt)) per platinum surface area at 0.9V under the following evaluation conditions.

[ solution 2]

< evaluation Condition >

Temperature: 80 deg.C

Gas composition: hydrogen (4L/min anode side)/oxygen (8L/min cathode side)

Relative humidity: 100% RH/100% RH

Pressure: 150kPa (abs)/150kPa (abs)

Voltage scan direction: measured from a voltage value of 10A to a voltage value of 0.2A

The results are shown in table 2 below.

[ Table 2]

As is clear from table 2, the catalyst of the present invention is excellent in oxygen reduction reaction activity.

Experiment 2: evaluation of Power Generation Performance

The power generation performance of the membrane electrode assembly (1) prepared in example 1 and the comparative membrane electrode assembly (3) prepared in comparative example 3 was evaluated by measuring the voltage (V) at 2.0A/cm2 under the following evaluation conditions.

[ solution 3]

< evaluation Condition >

Temperature: 80 deg.C

Gas composition: hydrogen (anode side 4L/min)/air (cathode side 15L/min)

Relative humidity: 100% RH/100% RH

Pressure: 200kPa (abs)/200kPa (abs)

experiment 3: evaluation of oxygen transport resistance

Oxygen transport resistance was evaluated for the membrane electrode assembly (1) produced in example 1 and the comparative membrane electrode assembly (3) produced in comparative example 3 according to the method described in t. mashio et al.ecs trans, 11,529, (2007).

That is, the limiting current density (A/cm2) was measured using diluted oxygen. At this time, the gas transport resistance (s/m) was calculated from the slope of the limiting current density (A/cm2) with respect to the oxygen partial pressure (kPa). In table 4, the oxygen transport resistance values obtained in comparative example 3 are shown as relative values of 1.

Experiment 4: effective surface area (ECA) maintenance ratio of catalyst to initial stage

The catalyst effective surface area (ECA) maintenance ratios of the membrane electrode assembly (1) produced in example 1 and the comparative membrane electrode assembly (3) produced in comparative example 3 were calculated from the ECA ratio of 100% relative humidity before and after the load cycle durability evaluation (the evaluation conditions are referred to below). The results are shown in table 3 below.

[ solution 4]

< evaluation Condition >

Temperature: 80 deg.C

Gas composition: hydrogen (0.5L/min anode side)/oxygen (0.5L/min cathode side)

Relative humidity: 100% RH/100% RH

Pressure: 100kPa (abs)/100kPa (abs)

Voltage: 0.6V-0.95V (1 cycle)

(holding at each voltage for 3 seconds, 50000 cycles were carried out)

[ Table 3]

As is clear from table 3 above, the MEA (1) using the catalyst of the present invention has excellent power generation performance as compared with the comparative MEA (3) not having the amount of acidic groups defined by the present invention. Further, it is found that the oxygen transport resistance in the catalyst layer is intentionally low, and the ECA maintenance ratio (durability of the metal catalyst in the catalyst layer) is excellent.

In addition, the present application is based on japanese patent application No. 2013-92923, filed on 25/4/2013, the disclosure of which is incorporated by reference in its entirety.

Claims

1. A cathode catalyst layer for a fuel cell, wherein,

A catalyst comprising a catalyst carrier and a metal catalyst supported on the catalyst carrier, and obtained by a production method comprising a step of obtaining a catalyst powder by supporting the metal catalyst on the catalyst carrier, and then heat-treating the catalyst powder in a hydrogen atmosphere,

The metal catalyst is supported in a cavity having a radius of 1nm or more of the catalyst carrier,

The catalyst has a specific surface area per unit weight of the carrier of 1200m2/g or more, and,

The amount of the acidic group per unit weight of the carrier in the catalyst is 0.75mmol/g or less per the carrier,

Comprises a proton-conductive polymer electrolyte, and the coverage of the metal catalyst with the proton-conductive polymer electrolyte is less than 0.5,

The metal catalyst supported in the cavities is in a non-contact state with the proton-conductive polymer electrolyte.

2. The cathode catalyst layer for a fuel cell according to claim 1,

The coverage of the proton-conducting polymer electrolyte with the metal catalyst is 0.15 or more.

3. The cathode catalyst layer for a fuel cell according to claim 1,

The amount of the acidic groups per unit weight of the carrier in the catalyst is less than 0.7mmol/g carrier.

4. The cathode catalyst layer for a fuel cell according to claim 1,

The amount of the acidic group per unit weight of the carrier in the catalyst is 0.6mmol/g or less per the carrier.

5. The cathode catalyst layer for a fuel cell according to claim 1,

the coverage of the metal catalyst with the electrolyte is less than 0.4,

The specific surface area per unit weight of the carrier of the catalyst is more than 1291m2/g of carrier.

6. The cathode catalyst layer for a fuel cell according to any one of claims 1 to 5,

The support contains a metal oxide or carbon.

7. The cathode catalyst layer for a fuel cell according to any one of claims 1 to 5,

The acidic group is at least one selected from the group consisting of a hydroxyl group, a lactone group and a carboxyl group.

8. The cathode catalyst layer for a fuel cell according to any one of claims 1 to 5,

The metal catalyst is platinum or contains platinum and a metal component other than platinum.

9. The cathode catalyst layer for a fuel cell according to any one of claims 1 to 5,

The loading amount of the metal catalyst to the carrier is 50% by weight or less.

10. The cathode catalyst layer for a fuel cell according to claim 1,

The specific surface area per unit weight of the carrier of the catalyst is 1700m2/g or more.

11. A membrane electrode assembly for a fuel cell, comprising the cathode catalyst layer for a fuel cell according to any one of claims 1 to 10.

12. A fuel cell comprising the membrane-electrode assembly for a fuel cell according to claim 11.

13. The fuel cell according to claim 12, which is a solid polymer fuel cell.